1. Field of the Invention
The present invention relates to a method of milking an animal by a milking machine having a teatcup with a teatcup liner and a pulsation chamber, a first pressure source for generating a first pressure, and a second pressure source for generating a sub-atmospheric second pressure, which is lower than the first pressure, the method comprising connecting the second pressure source to the interior of the teatcup liner, and alternately transmitting the first pressure from the first pressure source and the second pressure from the second pressure source to the pulsation chamber, to generate a pulsating pressure in the pulsation chamber, whereby the teatcup liner cyclically moves with a decreasing opening rate to an essentially open position during the transmission of the second pressure and with a decreasing closing rate to an essentially closed position during the transmission of the first pressure. Moreover, the invention relates to a milking machine comprising at least one teatcup having a teatcup liner and a pulsation chamber, a first pressure source for generating a first pressure, a second pressure source for generating a sub-atmospheric second pressure, which is lower than the first pressure, conduit means for transmitting the first pressure and the second pressure, respectively, from the first pressure source and the second pressure source, respectively, to the pulsation chamber, and a pulsator for alternately connecting the first pressure source and the second pressure source to the pulsation chamber via the conduit means, such that the teatcup liner cyclically moves with a decreasing opening rate to an essentially open position during the transmision of the second pressure and with a decreasing closing rate to an essentially closed position during the transmission of the first pressure.
2. Description of the Prior Art
A conventional milking machine comprises a cluster having a claw and four teatcups, each teatcup having a shell and a teatcup liner provided in the shell to form a pulsation chamber between the teatcup liner and the shell. During milking the interior of the teatcup liner is subjected to a sub-atmospheric pressure, i.e. normally about 50 kPa under atmospheric pressure. There are also milking machines working under high-pressure conditions, wherein the low pressure might be above atmospheric pressure. The pulsation chamber is subjected to a cyclically pulsating pressure normally varying between atmospheric pressure, when the teatcup liner is collapsed or closed and a minimum pressure level when the teatcup liner is fully open. The minimum pressure level is normally equal to said sub-atmospheric pressure level, i.e. 50 kPa under atmospheric pressure. This means that the pressure difference across the wall of the teatcup liner is essentially equal to zero when the teatcup liner is fully open.
The pulsating pressure demonstrates a pulsation cycle which may be divided into four phases, i.e. (a) an opening phase during which the pulsating pressure decreases from atmospheric pressure to normally said sub-atmospheric pressure prevailing in the interior of the teatcup liner and during which the teatcup liner moves from an essentially closed position to an essentially open position, (b) an open phase during which the pulsating pressure has reached its minimum level and is essentially equal to said sub-atmospheric pressure and during which the teatcup liner is in an open position, (c) a closing phase during which the pulsating pressure increases from about said sub-atmospheric pressure to the atmospheric pressure and during which the teatcup liner moves from the open position to the closed position, and finally (d) a closed phase during which the pulsating pressure is equal to the atmospheric pressure and the teatcup liner is in a closed position. The time relations between the open and closed positions are defined in the pulsator ratio. The opening and the closing of the teatcup liner during phase (a) and (c) respectively, comprises a very fast and abrupt movement of the teatcup liner. From a closed position, i.e. opposite wall portions of the teatcup liner touch each other, as the pulsating pressure decreases the teatcup liner remains essentially closed until it at a certain pulsating pressure level, the so called TPD-level (touch pressure difference), starts to open abruptly to the so called CCPD-level (critical collapse pressure difference) at which level the teatcup liner is fully open, i.e. said opposite wall portions of the teatcup liner are spaced apart from each other. From the point of time when the pulsating pressure is less than the CCPD-level the teatcup liner thus is essentially open and a further decrease of the pulsating pressure only results in an insignificant further opening of the teatcup liner.
Each milking may be divided into four periods, i.e. (I) the initial stimulation or massage period, (II) the main flow period, (III) the flow decreasing period, and (IV) the flow terminating period. During the initial, flow decreasing and flow terminating periods the milk flow is reduced in comparison with the main flow period.
Such conventional milking machines of today involve the following problems. During especially the flow decreasing and terminating period of each milking, when a small amount or no milk is flowing, the teats are fully subjected to the sub-atmospheric pressure prevailing in the interior of the teatcup liner. Additionally due to the opening and closing movements of the teatcup liner the volume beneath the teat will change. This volume change is compensated by air moving to the teat during the opening movement of the liner. The velocity of this moving air is very high (high jet flow). This air flow may carry milk droplets and bacteria. These droplets are accelerated and directed to the teat and are hit against the teat by the so called impact. Such high impact and high jet flow create a high risk for new infections, such as mastitis. It may happen that bacteria associated with one teat pass directly into the interior of another teat, and thereby increasing the risk of infection.
During high milk flow there appear cyclic pressure (vacuum) fluctuations beneath the teat tip due to the opening and closing movement of the teatcup liner. Such fluctuations can irritate the teats and stress the teat tissue, which negatively influences the udder health.
Moreover such pressure decrease (vacuum increase) due to the volume change in the interior of the teatcup liner during in particular the terminating period may also result in that the teatcup is crawling upwards on the teat. Such crawling at the end of the milking as the teat becomes slack leads to a throttling of the milk conducting interior of the teat, and consequently the milk flow may be interrupted although a considerable amount of rest milk remains in the udder.
These problems are all associated with the very fast and abrupt opening and closing movement of the teat cup liner. Therefore it has been proposed to slow down the opening and closing movement. However, slowing down the opening and closing movement of the teatcup liner also results in a time delay until the liner starts moving. Therefore, milk flow from the teats might be delayed in comparison to a teatcup liner working in the conventional manner. This problem which is of greatest significance during the main flow period, may lead to the milking operation as a whole being prolonged.
WO-A-9 212 625 discloses a method of milking an animal by using a milking machine having at least one teatcup with a teatcup liner and a pulsation chamber. The method of this document aims at a reduction of the rest milk and a more gentle treatment of the teats of the animal, and at the same time a rapid milk extraction should be maintained. To this end, the milking machine is operated to cyclically pressurize the pulsation chamber such that during each pulsation cycle the liner opens and/or closes at a slower rate during the initial and/or flow decreasing and terminating periods of the milking operation than during the main flow period of the milking operation. This method thus involves an extended a-phase or c-phase of each pulsation cycle.
U.S. Pat. No. 5,218,924 discloses a milking system in which the pulsator is replaced by a variable pressure source, in order to permit transmission of a controllably variable pressure to the pulsation chamber, i.e. it is possible to vary the rate of change of pressure supplied by the variable pressure source to the pulsation chamber. This document aims at a shorter transition time between the maximum pressure level and the minimum pressure level in the pulsation chamber. Thereby the liner movement is speeded up in comparison to conventional milking, such that this movement from a fully closed to a fully open liner and from a fully open to a fully closed liner, respectively, will occur within a very short time period of 0.05 seconds. Such a rapid transition movement is produced by an almost instantaneous pressure change in the pulsation chamber, followed by a somewhat slower rate of pressure change, which in turn is followed by a final almost instantaneous pressure change.